1. Field of the Invention
The invention disclosed herein relates generally to the circuit configuration and packaging configuration of power MOSFETs. More particularly, this invention relates to a novel device for preventing shoot through problem by adjusting the work function of the MOS gate.
2. Description of the Prior Art
Conventional power MOSFET devices still face the shoot through problems that result in excessive dissipation and efficiency loss. Referring to FIG. 1 for a circuit diagram of a conventional buck converter 10 that includes a high side MOSFET 15 and a low side MOSFET 20 serially connected between an input terminal 25 having an input voltage represented by Vin and a ground terminal 30. The drain of the low side MOSFET 20 is connected to the source of the high side MOSFET 15 at a mid point 35 connecting to the load 40 through inductance L and capacitance C. When the buck converter 10 operates at high speed, a shoot through condition becomes a problem when both the high side and low side MOSFET are turned on at the same time causing a shoot through current to flow between the input terminal 25 and the ground terminal 30. The shoot through condition results in excessive dissipation and efficiency loss. In order to avoid the shoot through problem, a controlling circuit 45 is implemented to control the gate signals to generate a dead time between the gate signals for the high side and low side MOSFET. FIG. 2 shows such a dead time between the time when the high side MOSFET 15 is turned off and the time when the low side MOSFET 20 is turned on such that the high side and low side MOSFETs are prevented from turning on simultaneously.
However, the shoot through problem cannot be completely avoided due to the fact that a large drain current is generated at the low side MOSFET 20 when the high side MOSFET 15 is turned on as shown in FIG. 3 due to a large rate of change of the voltage, i.e., dV/dt, at the mid-connection point 35. FIG. 4A shows an equivalent circuit of the buck converter wherein the drain current generated flows through the gate-drain capacitor Cgd and then to the ground through the internal gate-source capacitor Cgs or through an equivalent circuit segment comprises gate resistor Rg inductor Lg, and external gate drive resistance Rext. Under such circumstances, if the impedance from the gate to the ground is not below a certain value then the drain current, i.e., Cdg*dV/dt, will generate a voltage drop across the gate of the low side MOSFET that would be large enough to turn on the low side MOSFET 20 thus inducing shoot-through.
In a full bridge application such as shown in FIG. 4B, a DC power supply Vin drives an inductive load L. In one half-cycle, both Q1 and Q4 are on and both Q2 and Q3 are off, the current flows through Q1 to the load L and through Q4 to the ground. In the next half-cycle both Q2 and Q3 are switched on and Q1 and Q4 are off, the current flows through Q2 to the load L and through Q3 to the ground. At the moment Q2 is turned on Q4 must be completed off. However, large rate of change of the voltage over Q2 when Q2 is turned on may force Q4 to turn on as described above and cause shoot-through.
In modern circuit designs, a designer typically controls the problem by using a large gate-source capacitance Cgs or a low Crss/Ciss ratio where the input capacitance Ciss and feedback capacitance Crss are determinate by the follows:Ciss=Cgd+Cgs Crss=Cgd Alternately, the problem may also be prevented by providing a low gate resistance and using a high current gate drive with low Rext. However, if the gate drive circuitry, i.e., the control circuit 45, is remote from the MOSFET, the inductance Lg may become quite large. This causes the current path connected with Rg, Rext, and Lg to have great impedance thus leaving only the Cgs path to sink the transient current. The only way to suppress the shoot through current is by increasing the capacitance Cgs to reduce the impedance. However, this solution will lead to excessive gate charge losses in the low side MOSFET 20. For the above reasons, a person of ordinary skill of the art is faced with limitations and difficulties in designing a converter to effectively prevent the shoot through problem.
FIG. 5A shows a typical conventional trench MOSFET. As illustrated in this trench MOSFET cell, the gate-drain capacitance, i.e., Cgd, is a combination of a series the oxide capacitance, i.e., Cox and the depletion layer capacitance, i.e., Cdep. A functional relationship can be expressed as:Cgd=Cox*Cdep/(Cox+Cdep)As shown in FIG. 5B, the gate-drain capacitance Cgd decreases rapidly with the drain to source bias voltage, i.e., Vds bias, due to the decrease in depletion capacitance, i.e., Cdep, with the increase in depletion width under high bias. When the drain to source bias Vds is small, the oxide capacitance Cox dominatingly determines the value of the capacitance Cdg. And, since this capacitance Cox can be quite high and therefore can likely contribute to the shoot through current. In order to resolve these problems, different cell configurations were available to reduce the oxide capacitance Cox. FIG. 6A shows a prior art trench MOSFET with a thick bottom oxide that helps to reduce the capacitance from gate to drain Cgd. This cell configuration further provides additional benefits of reducing the capacitance across the entire range of Vds voltages. FIG. 6B shows another conventional solution where a second electrode is inserted below the gate electrode and tied to source potential. This electrode shields the gate from the drain thus reducing the gate to drain capacitance Cdg. However, both of these configurations add significant complexity to the fabrication processes. Furthermore, additional complication arises due to the facts that control of the recessed electrode and oxide is quite difficult.
Therefore, a need still exists in the art to provide an improved device configuration and manufacturing methods to make MOSFET devices with a work-function of the gate to shift the capacitance-voltage (C-V) functional characteristic to prevent the occurrences of shoot-through and resolve the above discussed difficulties as now encountered in the prior art.